Electroplating bath



Patented Feb. 24, 1925.

UNITED STATES PATENT OFFICE.

ISAAC M. SCOTT AND SAMUEL PEACOCK, OF WHEELING, WEST VIRGINIA, WIL- LIAMEARL ARMSTRONG, OF MARTINS FERRY, OHIO, ASSIGNORS' T WHEELING STEEL &IRON COMPANY, 0F WHEELING,

WEST VIRGINIA.

WEST VIRGINIA, A CORPORATION OF ELECTROPLATING BATH.

No Drawing.

To all whom it may concern Be it known that we, IsAAo M. Soo'rT, SAMUELPEACOCK, and WILLIAM EAR ARM- STRONG, all citizens of the United States,the

said IsAAo M.- Soom and SAMUEL PEACOCK residing at Wheeling, in thecounty of Ohio and State of West Virginia, and the said lVILLIAM EARLARMSTRONG residing at Martins Ferry, in the county of Belmont and Stateof Ohio, have invented certain new and useful Improvements inElectroplating Baths; and we do hereby declare the following to be afull, clear, and exact description of the invention, such as will enableothers skilled in the art to which it appertains to make and use thesame.

This invention relates to improvements in electro-tinning and moreparticularly to the solution of electrolyte, whereby said electrotinningcan be carried out on a commercial scale and in a manner more economicalthan has been heretofore attained.

With these and other objects in view the invention consists in the novelcomposition of matter constituting the electrolyte, all as will be morefully hereinafter disclosed and particularly pointed out in the claims.

In order that the precise invention may be the more clearly understoodit is said :-In the electro-deposition oftin, great difficulty has beenheretofore experienced in producing a dense and coherent deposit. Thisis especially emphasized when the objects to be electro-tinned areconstructed of metals higher in the electrochemical series than is tin,such for example, as steel or iron. Here-' tofore, articles constructedof metals such as iron, were first electroplated with a metal lower inthe electro-chemical series such for example as copper, and upon thislast named metal the tin was deposited. But, solutions that areavailable for depositing the tin directly on iron or steel deterioraterapidly, and cannot be operated on a commercial scale, economically,unless associated with a suitable restoring solution. These difficultiesare all well known in this art.

On the other hand, this invention involves a solution and a method bywhich electro- 59 tinning can be carried out without difiiculty and muchmore efliciently than heretofore; and this solution can be used forplating Application filed February 19, 1923. Serial No. 620,010.

directly on steel and other metals higher in the 'electro-chemicalseries. Further, this method may be used for large scale operations withdifl'erent concentrations and without causing a formation of colloidalor spongy deposits as in the prior procedures. The solutions contain alower percentage of tin than those heretofore employed, they do notdeteriorate or hydrolize to an objectionable extent, and they operatewith almost 100% efliciency. This bath does not require a periodicaladdition of tin salts as heretofore, 01' in other words, it is selfsustaining.

It will be understood by those skilled in the art that the thickness ofdeposit or amount of tin plated per' unit of surface will depend uponthe length of time and the number of amperes used, irrespective of thevoltage, but of course, the lower the voltage employed the moreeconomical will be the deposition.

It has been found in practice that with say to 1% volts good depositsare obtained with this solution. In carrying out the invention initspreferred form, one may take, for example, 3 parts by weight of acommercial hydrofluoric acid (HF) and dissolve therein 1.6 parts byweight of boric acid H BO These acids react with an evolution of heatand form an acid known as fluoboric acid or borofluohydric acid, thesimplest formula of which is HBF In this acid, one dissolves stannousoxide SnO until the acid has taken up the equivalent of 1.35 parts byweight of metallic tin, leaving about 31% of the acid present in thefree state.

By this means, there is produced a solution of stannous fluoborateSn(BF,) containing an excess of free fluobori-c acid HBIQ. To increasethe conductivity of this solution one adds a metallic salt of a metalhigher in the electro-cliemical series than iron, and prefers for thispurpose the addition of 1.3.parts by weight of aluminum sulphate Al (SO3 or 1.3 parts by weight of magnesium sulphate MgSO,,. This solution isthen diluted with water to such a concentration that it will containabout 1 ounces of metallic tin to one gallon. In this diluted solutionthere is suspended metallic tin as an anode on a positive bus bar-andone suspends the articles to be lated such as sheet steel in thesolution an connects them to the negative bus bar whereby on the passageof a suitable electric current, the steel articles become plated withmetallic tin.

It has beenfound it is also desirable to add so-called addition agentsor brightening agents, such as mollasses, gelatine, etc., to thesolution for the purpose of obtaining somewhat improved and brightdeposits. It is preferred to use ordinary glue for this purpose and in aquantity of about .03 ounce per gallon of solution, but a larger amountwill not interfere with the success of the process. Instead of usingstannous oxide $110 for introducing the tin into this solution, one mayadd other tin salts, such as stannous hydrate Sn(OH) or may dissolve inthe solution metallic tin with or without the aid of the electriccurrent. It has been found that the economical addition of tin may alsobe obtained by suspending metallic tin as anodes in the solution,connected with a positive bus bar and suspending metal strips,surrounded by porous cups in the solution, the metal strips beingconnected to a negative bus bar. By the passage of a suitable electriccurrent, such as that above disclosed, the tin is dissolved from theanodes into the solution and being unable to pass into the porous cups,it is taken up by the acid. When suflicient tin has been dissolved thecups are removed and the plating continued.

We are unable to satisfactorily explain the exact scientific reasons whythis solution produces such surprising efiiciencies in the coating oftin onto iron, but a plausible explanation resides in the followingfacts It is well known that the usual bath for electro-depositing tincontains the tin in the form of a sodium stannite or a sodium stannate.It is doubtful if a sodium stannate is in any way useful for tindeposition on iron. It is also well recognized that potash salts areequally inetlicient with sodium salts in the electrodeposition of tin oniron. But while these alkali metal tin baths give good initial deposits,they very rapidly l1y-.

drolize in practical use so that after a very small percentage of tinhas been deposited a satisfactory coating is no longer obtained. The tincontent of these prior baths hydrolizes in various forms but it isgenerally accepted that said content separates out in the form of aninsoluble meta stannic acid. There is, however, some question as to whatis the real form this said insoluble tin actually takes, but there is nodispute as to the fact that it separates out of the solution in aformthat makes it no longer available for elcctro deposition purposes.

Another fact which makes the alkali tin baths unsatisfactory is that theanode efficiency is very low.

That is, the tin is removed from the solution at an efficiency which isnowhere near a theoretical efiiciency if we consider the quantitydissolved from the anodes. It results that for any considerable quantityof tin deposited by the old procedures, fresh additions of alkali tinsalts must constantly be added. 'It is therefore evident that it is onlya question of time when the concentration of caustic alkali in thesolution will be so great as to preclude any further deposition of tinand this fact alone constitutes a serious objection to the priorprocedures.

It is a further recognized fact that with tin depositing salts, in whichthe tin exists as a base, such for example, as the chlorides or thesulphates, there is also experienced a hydrolytic separation of the tinin forms that are insoluble, and therefore not ionized. It thus againfollows that these salts are not utilizable for a satisfactoryelectrodeposition of metallic tin. Unless these salts are maintained ina very acid solution, this hydrolytic decomposition is so rapid as topreclude the use of such baths for deposition purposes. But on the otherhand, if the solution is maintained rather strongly acid, it isimpossible to deposit satisfactory coherent tin coatings on metalliciron or steel, because of the action of the acid upon the iron or steelarticles. That is, although the deposit may be made, it is very spongyand does not adhere.

There have also been tin baths proposed in which cyanides are employed,and while these baths serve somewhat better than the alkali tin saltsfor depositing on iron, yet, they are dangerous in practical operations,because stannic acid is capable of setting free prussic acid which isvery poisonous and highly dangerous to the operatives. But even if thisdanger were obviated for which no means are known at this time, the bathis still unsatisfactory in practice due to an excessive hydrolyticdecomposition of the tin.

It has further been proposed to use baths of tin containing alkalifluorides. While these baths have a somewhat higher anode efficiencythan the baths in which tin is present as a base, yet, they still failto deposit in large scale operations on iron or steel a satisfactoryfirmly adherent reguline deposit and are also subject to a veryobjectionable bydrolytic dissociation.

The bath composed of stannous oxide dissolved in fluoboric acid on theother hand is found to be largely free from the above objections, dueprobably to the presence of both the boric and the fluoric ions, and sofar as is known seems to be the only bath which is not accompanied by anobjectionable hydrolytic dissociation. This feature enables this bath todeposit tin on iron and steel in a firmly adherent, reguline coating,which practice proves to be very satistory indeed.

In the foregoing description there is discloseda tin solution in asomewhat diluted form with an acid reaction, but the solution can beused in more concentrated forms such for example as in those having aplurality of times the concentration of that above mentioned. Thesolution can also be used in the neutral state and for certain purposesone prefers a neutral solution, and such last named solution isespecially desirable when chemically pure tin is being deposited.

The above solution has proved satisfactory for the deposition of tin onvarious metals such as iron, steel and copper, and therefore, it is notdesired to be limited to the procedure set forth in the description.This solution and rocess may also be used with various modifications fornumerous purposes that will readily suggest themselves to those skilledin the art without departing from the spirit of our invention.

For example, a conducting metal salt, carbon or other electric conductormay be deposited byvwell known means in the pores of a fabric or othermaterial and tin may be deposited thereon by following the foregoingprocedure. Therefore, it is not desired to be limited to the abovedisclosure except as may be required by the claims.

What is claimed is 1. The herein described electro-plating bath fordepositing tin on electric conductors the same consisting essentially ofa solution of stannous fluoborate.

2. The herein described electro-plating bath for depositing tin onelectric conductors the same consisting essentially of a solution ofstannous fluoborate admixed with the salt of a metal electro-positive toiron.

3. The herein described electro-plating bath for depositing tin on ironor steel the same consistin of a solution of stannous .fluoborateadmixed wlth a salt of a metal

